Aromatic hydrocarbon compounds such as benzene, toluene, and xylenes (“BTX”) are frequently used for producing transportation fuels and petrochemicals such as styrene, phenol, nylon, polyurethanes, and many others. Processes have been developed for producing aromatic hydrocarbon from relatively inexpensive feeds, e.g., from paraffinic C4-feeds. The processes typically are carried out using a catalyst comprising molecular sieve, such as ZSM-5 and at least one dehydrogenation metal. Aromatic hydrocarbon can be recovered from unreacted feed and reaction byproducts in an aromatics recovery system located downstream of the aromatization process. Yield of aromatic hydrocarbon has been found to be limited by the presence of molecular hydrogen produced during hydrocarbon dehydrogenation. Aromatic hydrocarbon yield has also been found to be limited by side reactions such as hydrogenolysis and coking. Hydrogenolysis undesirably produces a light saturated hydrocarbon byproduct. Coking produces undesirable catalyst deposits, which decrease catalyst activity and increase reactor pressure drop.
Light hydrocarbon is often available in combination with CO2, and these resources have generated considerable interest as potential feeds to aromatization processes. Although CO2 can be separated from the feed upstream of the aromatization, this approach is typically undesirable in view of the inefficiency and complexity of the refrigeration equipment used to do so. CO2 can also be separated downstream of the aromatization, but doing so can bottleneck the aromatic hydrocarbon recovery system. These difficulties can be at least partially overcome by converting CO2 to CO and water during the aromatization. As reported in Yamaouchi et al, Sekiyu Gakkaishi, 37, (3), 278-284 (1994), aromatization of a feed comprising propane and CO2 can be carried out using a catalyst comprising ZSM-5 loaded with Zn, Ga, or Pt. The reference reported an increase in aromatic hydrocarbon yield at a reaction temperature of about 823° K, which was attributed to the beneficial effects of a reverse water gas shift reaction (“RWGS”): CO2+H2→CO+H2O. Particularly with Pt-ZSM-5 catalysts, the reference reported an improvement in the yield of aromatic hydrocarbon resulting from molecular hydrogen conversion in the reverse water gas shift reaction. The reference also reports an undesirable hydrogenolysis increase during simultaneous aromatization and RWGS, which can bottleneck the aromatics recovery system. Increased methane production from hydrogenolysis has also been found to increase hydrocarbon partial pressure during the aromatization, leading to an increase in the rate of catalyst coking. Although the amount of hydrogenolysis can be mitigated (e.g., by adding an attenuating metal to the catalyst as disclosed in U.S. Pat. No. 8,692,043), this significantly increases catalyst complexity.
It is also observed that light hydrocarbon aromatization and RWGS are endothermic reactions. Combining these two reactions for light hydrocarbon aromatization in the presence of CO2 for increased aromatic hydrocarbon yield may also increase reactor complexity and decrease the process's energy efficiency. For example, the aromatization reactor vessel may need modification so that additional heat can be added to support the RWGS reaction. The additional heat can be produced by combustion of a fuel, but at an increased energy debit over the energy needed for the aromatization reaction.
There is therefore a need for improved processes and catalysts for aromatization of substantially non-aromatic hydrocarbon in the presence of CO2.